Scientists are Starting to Understand Solar Cycles

The Sun taken by SOHO on Feb. 10, 2006. Image credit: SOHO Click to enlarge
Solar scientists think they’re finally getting a handle on predicting the Sun’s cycles. If everything goes as they predict, the next solar cycle will be 30-50% stronger, and be up to a year late. Astronomers have been tracking the two major flows of plasma that goven the Sun’s cycles. One acts like a conveyor belt, pulling plasma from the poles to the equator, and the other gets stretched since the Sun rotates faster at the equator than at the poles. This causes the Sun’s magnetic field to concentrate, creating the solar maximum.

Scientists predict the next solar activity cycle will be 30 to 50 percent stronger than the previous one and up to a year late. Accurately predicting the sun’s cycles will help plan for the effects of solar storms. The storms can disrupt satellite orbits and electronics; interfere with radio communication; damage power systems; and can be hazardous to unprotected astronauts.

The breakthrough “solar climate” forecast by Mausumi Dikpati and colleagues at the National Center for Atmospheric Research in Boulder, Colo. was made with a combination of computer simulation and groundbreaking observations of the solar interior from space using NASA’s Solar and Heliospheric Observatory (SOHO). NASA’s Living With a Star program and the National Science Foundation funded the research.

The sun goes through a roughly 11-year cycle of activity, from stormy to quiet and back again. Solar storms begin with tangled magnetic fields generated by the sun’s churning electrically charged gas (plasma). Like a rubber band twisted too far, solar magnetic fields can suddenly snap to a new shape, releasing tremendous energy as a flare or a coronal mass ejection (CME). This violent solar activity often occurs near sunspots, dark regions on the sun caused by concentrated magnetic fields.

Understanding plasma flows in the sun’s interior is essential to predicting the solar activity cycle. Plasma currents within the sun transport, concentrate, and help dissipate solar magnetic fields. “We understood these flows in a general way, but the details were unclear, so we could not use them to make predictions before,” Dikpati said. Her paper about this research was published in the March 3 online edition of Geophysical Research Letters.

The new technique of “helioseismology” revealed these details by allowing researchers to see inside the sun. Helioseismology traces sound waves reverberating inside the sun to build up a picture of the interior, similar to the way an ultrasound scan is used to create a picture of an unborn baby.

Two major plasma flows govern the cycle. The first acts like a conveyor belt. Deep beneath the surface, plasma flows from the poles to the equator. At the equator, the plasma rises and flows back to the poles, where it sinks and repeats. The second flow acts like a taffy pull. The surface layer of the sun rotates faster at the equator than it does near the poles. Since the large-scale solar magnetic field crosses the equator as it goes from pole to pole, it gets wrapped around the equator, over and over again, by the faster rotation there. This is what periodically concentrates the solar magnetic field, leading to peaks in solar storm activity.

“Precise helioseismic observations of the ‘conveyor belt’ flow speed by the Michelson Doppler Imager (MDI) instrument on board SOHO gave us a breakthrough,” Dikpati said. “We now know it takes two cycles to fill half the belt with magnetic field and another two cycles to fill the other half. Because of this, the next solar cycle depends on characteristics from as far back as 40 years previously – the sun has a magnetic ‘memory’.”

The magnetic data input comes from the SOHO/MDI instrument and historical records. Computer analysis of the past eight years’ magnetic data matched actual observations over the last 80 years. The team added magnetic data and ran the model ahead 10 years to get their prediction for the next cycle. The sun is in the quiet period for the current cycle (cycle 23).

The team predicts the next cycle will begin with an increase in solar activity in late 2007 or early 2008, and there will be 30 to 50 percent more sunspots, flares, and CMEs in cycle 24. This is about one year later than the prediction using previous methods, which rely on such statistics as the strength of the large-scale solar magnetic field and the number of sunspots to make estimates for the next cycle. This work will be advanced by more detail observations from the Solar Dynamics Observatory, scheduled to launch in August 2008.